Marine finfish aquaculture uses pens to contain the finfish and to prevent access by predators. A typical pen consists of a net suspended at the surface by a floating structure (e.g., a circular plastic collar). The walls of the net extend vertically from the water's surface to a depth of typically 6 to 20 meters, and then across the bottom of the pen.
Over the past 30 years, the size of containment pens has increased. Early containment pens were 40 meters in circumference (or 15 meter square). Today's standard containment pen is 100 meters in circumference, with a volume of 6,000 to 20,000 cubic meters. A single aquaculture farm operation can have 30 of these circular pens.
The nets used in these pens are huge. Dry, they weigh several tons and at the end of a growing cycle they can weigh 20 tons or more owing to fouling from marine organisms such as algae and mussels. The handling of these huge nets for repair and maintenance has become a huge logistical and economic problem for fish farmers. Too bulky and heavy to handle, nets must be cut off the pens, often ending up on the ocean bottom where, according to newly issued discharge permits, they must be recovered.
Suspended nets are subject to deformation from currents, sagging, and bagging. This can result in pockets where predators such as seals can push in to bite fish, with resulting tears to the net. Therefore, these pens need a predator net, a secondary, coarser net entirely encapsulating the containment net. The predator net is typically suspended from the outside of the floating circular collar, and the containment net from the inside of the collar, to keep a one meter separation between predator and containment nets. This is generally believed to be sufficient to keep predators from pushing into the containment net.
A third net has to be incorporated into the current technology—a bird net is suspended above the surface pens to prevent access to the pens by predator birds, e.g., osprey, eagles, herons, and gulls.
Suspended net systems are generally located nearshore, where sea conditions (e.g., waves and winds) are less severe, and access is easier. But offshore locations are known to have advantages. Water depths are greater, thus permitting the containment pens to be submerged during storm conditions when fish would naturally move to greater depths to avoid damage from wave action. Water quality is generally better, as there is less pollution from land sources. The containment pens have a much reduced aesthetic presence, and thus face less political opposition. Greater water circulation through the pens helps dilute waste products. Water temperatures are more stable. Risk of disease is reduced due to increased distance between farms.
Notwithstanding the known advantages of offshore locations, the art has not yet found a practical design for offshore containment pens, although various ideas have been advanced.
Willinsky U.S. Pat. No. 5,251,571 shows an offshore containment pen in the shape of a geodesic sphere formed of hubs and interconnecting struts. Two hemispheric nets are attached to the interior of the sphere, by attaching the net at many points (col. 4, lines 3-18). The sphere can be lowered below the ocean surface, and it can rotated at the surface using an axle and buoyant elements incorporated into the sphere.
Zemach U.S. Pat. No. 5,412,903 proposes a metal skeleton with a superimposed netting covering the skeleton.
Bones U.S. Pat. No. 5,628,279 shows fish cages designed to be raised and lowered along the submerged support columns of offshore oil platforms. The pens rely on injection-molded, fiberglass-reinforced grating panels painted with antifouling paint. The grating panels are supported in a rigid, generally hexagonal structure. An optional net may be installed if the fish are too small to be contained by the grating panels. Loverich U.S. Pat. No. 7,617,813 (Ocean Spar Technologies) discloses a submersible pen having a central vertical spar and a peripheral ring, with a net stretched around the peripheral ring to the two ends of the spar. Similar structures are in use in various locations, and pens as large as 3,000 cubic meters have been constructed. E.g., the Submersible Sea Station from Net Systems.
Sadco, a Russian company, has proposed a submerged metal cage from which a net is hung.
Another approach to offshore pens has been to strengthen the design of surface pens. E.g., the Dunlop Tempest pen uses flexible rubberized flotation collars filled with gas at high pressure supporting gravity nets. The pens have been used in Ireland for salmon, and have survived substantial storms. The Ocean Spar pen by Net Systems uses tensioned vertical steel spars supporting gravity nets. Farmocean proposes a galvanized steel superstructure supporting gravity nets.
Another issue in the design of containment pens is moving fish between pens or harvesting fish from pens. Conventional approaches are extremely labor intensive, and stressful for the animals. Workers use a seine net to crowd the fish to one side of a pen and then use a fish pump or brail net to move the fish. Often many sets of the seine are needed to capture the fish, and this operation cannot be done in rough weather. The fish are stressed from crowding, and often physically injured (scale loss, bruising, loss of protective slime) from the nets and the fish pump. The injuries lead to eventual mortality or downgrading at harvest.